The isomerization of olefins is catalyzed by a variety of agents including bases, acids, and a plethora of transition metal complexes. These catalysts yield a thermodynamic mixture of isomeric olefins if the isomerizations are allowed to proceed to equilibrium. Non-thermodynamic distributions of products are obtained if equilibrium is not achieved, but it is rare that olefin isomerization leads to high yields of one isomer unless it is highly favored thermodynamically.
As there is considerable commercial interest in transformations of olefin-esters to their .alpha.,.beta.-unsaturated esters, particularly the transformation of esters of oleic acid, it would be highly desirable to perform the isomerization in Equation 1. ##STR1##
This is a particularly challenging isomerization because of the many olefin isomers that are possible within the seventeen-carbon hydrocarbon chain. Because none of these isomers is highly favored thermodynamically, the .alpha.,.beta.-unsaturated ester isomer has been prepared by very elaborate methods. See, for example, Howton, Org. Prep. Proc. lnt., 6, 175, 1974; Barve et al., Chem. Phys. Lipids, 7, 311, 1971, Grimmer et al., Lieb. Annt. Chem., 685, 154, 1965; and Palameta et al., Tetrahedron, 19, 1463, 1963. A less complex method for such syntheses would be highly desirable. A less complex method that would promote the isomerization in Equation 1 could presumably also be used for the isomerization of other mono-ene esters to .alpha.,.beta.-unsaturated esters. Such reactive products would be useful intermediates for the synthesis of other new compounds.
Iron carbonyls have been used extensively as catalysts for the isomerization of olefins. For example, 1-pentene isomerizes to give an equilibrium mixture of cis-2-pentene (21%) and trans-2-pentene (76%) in the presence of Fe(CO).sub.5 in C.sub.6 H.sub.6 solvent under ultraviolet photolysis (Schroeder et al., J. Am. Chem. Soc., 98, 551, 1976). The same product distribution is obtained when Fe(CO).sub.3 (cis-cyclooctene).sub.2 catalyzes 1-pentene isomerization without photolysis (Fleckner et al., J. Am. Chem. Soc., 106, 2027, 1984). A mixture of isomers is obtained from the Fe.sub.3 (CO).sub.12 -catalyzed isomerizations of n-hexenes, n-pentenes, and methyl-1-pentenes under reflux (Manuel et al., J. Org. Chem., 27, 3941, 1962). Similarly, Fe(CO).sub.5 catalyzes the isomerization of n-alkenyl- and cyclohexenylcarboxylate esters and ethers to a mixture of isomers when refluxed (125.degree.-150.degree. C.) in hydrocarbon solvents or photolyzed at 20.degree. C. (Damico, J. Org. Chem., 33, 1550, 1968). Methyl oleate itself has been reported to give a very complex mixture of olefin isomers when heated at 185.degree. C. with Fe(CO).sub.5 ; however, in this mixture the amount of the .alpha.,.beta.-ester isomer was too small to be detected (Frankel et al., J. Am. Oil Chem. Soc., 43, 307, 1966).
Not only do the iron carbonyls catalyze olefin isomerization but there is evidence that under the proper conditions they might weakly stabilize the .alpha.,.beta.-ester isomer (Equation 1) by coordinating it as an .eta..sup.4 -oxadiene to Fe(CO).sub.3 as in A. ##STR2## Isolable Fe(CO).sub.3 complexes of .eta..sup.4 .alpha.,.beta.-unsaturated aldehydes (RCH.dbd.CH--C(.dbd.O)H), ketones (RCH.dbd.CH--C(.dbd.O)R') and imines (RCH.dbd.CH--C(.dbd.NR')H) are well known. The olefin-esters, .eta..sup.4 methyl acrylate and methyl trans-crotonate, also form .eta..sup.4 complexes, Fe(CO).sub.3 (.eta..sup.4 -H.sub.2 C.dbd.CHCO.sub.2 Me) and Fe(CO).sub.3 (.eta..sup.4 -CH.sub.3 CH.dbd.CHCO.sub.2 Me), that have been detected in ultraviolet-irradiated, low temperature matrices (10-12 K) containing Fe(CO).sub.5 and the .alpha.,.beta.-unsaturated esters. Even at -5.degree. C., Fe(CO).sub.3 (.eta..sup.4 -H.sub.2 C.dbd.CHCO.sub.2 Me) has been observed in n-hexane where it is in equilibrium with Fe(CO).sub.3 (.eta..sup.2 -H.sub.2 C.dbd.CHCO.sub.2 Me).sub.2, whose concentration depends on the concentrations of the complex and added methyl acrylate.
Although such studies suggest that it may be possible to stabilize the .alpha.,.beta.-ester isomer of methyl oleate by .eta..sup.4 coordination to Fe(CO).sub.3 if the complex (of type A) is produced at relatively low temperatures, this would mean that Fe(CO).sub.5 must catalyze the isomerization at low temperature. However, Fe(CO).sub.5 is known to be inactive as an olefin isomerization catalyst at low temperatures (e.g., at temperatures below about 0.degree. C. Furthermore, although at high temperatures (e.g., at temperatures greater than about 125.degree. C.), Fe(CO).sub.5 is an olefin isomerization catalyst, the .alpha.,.beta.-ester isomer cannot be stabilized by .eta.4 coordination to Fe(CO).sub.3 because the complex of type A is not stable at such high temperatures.